1. Field of the Invention
This invention relates to heating circuits which are particularly, but not exclusively, suitable for heating electric blankets and pads.
2. Description of the Prior Art
U.S. Pat. No. 3,461,585 discloses a heating circuit that comprises input terminals for connection to an AC supply, an elongate heating conductor and switch means connected in series between the input terminals, switch control means operative to close the switch means to cause current to flow through the heating conductor, a sensor conductor substantially coextensive with the heating conductor, and temperature sensitive means coextensive with the heating and sensor conductors and having an impedance that falls with increasing temperature.
In the known heating circuit, normal temperature control (i.e. control of the heat output under normal conditions) is provided by a thermistor responsive to ambient temperature to cause the switch control means to vary the firing angle of the switch means, which is a thyristor. Normal temperature control thus does not take account of the temperature of the temperature sensitive means. Overtemperature control, i.e. the prevention of current flow through the heating conductor in the event of overheating, is obtained by virtue of the impedance of the temperature sensitive means being connected in parallel with means for applying pulses to the thyristor to prevent the application of pulses to the thyristor when the impedance falls due to overheating. A disadvantage of this overtemperature control arrangement is that it is not very fail-safe. For instance, should the circuit go into a failure mode in which the switch means becomes permanently closed, current will continue to flow through the heating conductor, leading to a severe risk of damage, fire and personal injury. This risk is particularly severe when the heating circuit is embodied in an electric blanket or pad, in that the heating conductor is close to the body of the user and/or to a bed, whereby in the worst case the user could be seriously injured or even killed by fire or electrocution.